The use of “bent pipe” satellites for two way communications is well known. While there are many interesting topologies, a common one is the TDMA star topology. In this arrangement, there is a central station (often referred to as the “hub”) and multiple remote stations known as “remote terminals”. The remote terminals can number anywhere from a few terminals to hundreds of thousands of terminals or more. In the star topology, the remote terminals communicate with the hub, and data flow occurs from hub to remote terminal and vice versa.
Networks with the star topology have a single continuous carrier from the hub which is received by all the remote terminals. (The same hub location often supports multiple such carriers, but typically a given remote terminal will only listen to one or two such carriers.) This carrier is sometimes referred to as the “outbound” carrier. Transmissions from the remote terminals to the hub are “inbound”.
As the need for bandwidth increases, satellites have been designed to operate at higher and higher frequencies. As the frequency increases, the susceptibility to signal fading due to rain increases drastically. A link operating at 6 GHz may only need to accommodate a few dB of fade; this can easily be dealt with by overdesigning the satellite dish at the hub. However, as frequencies increase to 30 GHz, fades of tens of dB can occur. At these fade levels it is no longer practical to compensate by overdesigning the hub side RF equipment, and a strategy known as rain diversity must be used to preserve the link during rain fade.
Rain diversity is a known technique in which two different hub antennas are placed far enough apart so that the rain fade at the two sites are not significantly correlated. While there is some probability that there is significant rain over both sites, this probability is much smaller if the distance between the sites is sufficiently large. The required distance depends on the local geography, elevation angle to the satellite, and transmission frequency, but is on the order of tens of kilometers or more.
Traditional rain diversity for satellite earth stations (particularly hubs of TDMA networks) use L-band over analog fiber to connect a single set of networking equipment to two different antenna systems, with an analog switch to control the selection of the active antenna.
Such a conventional L-band switching approach may provide a single interface to a terrestrial data network, as well as providing a “home” for protocol enhancement endpoints.
A satellite station (either the hub or remote terminal) has a functional block called a “modem”. The purpose of the modem is to convert analog signals into digital data, and to convert digital data into analog signals. The analog signals are transmitted to, and received from, the satellite. The modem may be integrated into other hardware, and the transmit and receive functions of the modem may be split into separate functional blocks.
A typical prior art implementation of hub rain diversity involves converting the electrical analog signals into modulated analog signals on an optical fiber. While the signal is on optical fiber, it can travel long distances before being converted back to an electrical analog signal. This approach is illustrated in FIG. 1.
In FIG. 1, a satellite 100 is in communication with a primary site 102, diversity site 108, and user terminals 110 and 112. The primary site includes antenna 102a, L Band Switch 102b, L Band to Fiber Optic Converter 102c, transmit modem 102d and receive modem 102d. The diversity site includes an antenna 108a and L Band to Fiber Optic Converter 108b. In the primary site 102, the hub modem is split into the hub transmit modem 102d and a hub receive modem 102e, which are both controlled by a Hub Data Processing unit 104. The Hub Data Processing unit 104 is further connected to a Terrestrial Network Interface 106. According to some embodiments, the terrestrial network interface 106 is the connection between the remote terminals and the “outside world”. For example, if a user at a terminal wanted to browse www.yahoo.com, the data would flow through the terrestrial network interface. Furthermore, while the terrestrial network interface 106 does not directly participate in the rain diversity switchover, the interface to the system, as seen from the terrestrial network interface 106, does not change when a switchover occurs.
The output of the hub transmit modem 102d is converted to an optical signal, and is transmitted over the analog optical fiber 114 to the diversity site 108. At the diversity site 108 it is reconverted to an electrical analog signal, which is passed to the RF electronics for frequency conversion, amplification, and transmission towards the satellite. The reverse path applies to signals received from the satellite. A switch is used to select either the primary antenna 102a or the diversity antenna 108a. This switch can be set either manually, or automatically based on measured fade conditions or other criteria.
This prior art solution has two significant weaknesses:                1) Because the signal is analog, there is a limit to how far it can be transmitted (even with intermediate amplification stages) before the signal quality degrades to the point of impacting the satellite link quality. In the current state of the art, this range is on the order of 100 km or less. Based on the local geography and weather patterns, the range may be insufficient to ensure the rain fades of the two sites are sufficiently uncorrelated. In addition, choosing a hub site has other constraints, including the cost of land, regulatory environment, local zoning laws, access to high bandwidth terrestrial data networks, etc. These constraints may make it difficult to find a suitable diversity site within the limited range of the diversity solution.        2) The solution requires a dedicated fiber run. This involves either finding a preexisting unused fiber that happens to run between the hub sites, which may not be convenient, or laying one specifically for the application, which may be expensive.        